Electron guns

ABSTRACT

In a field emission electron gun means are provided for monitoring the number of electrons that pass through the hole in the first anode of the gun and form the useful beam, these means being used to control the cathode emission so as to keep the useful beam current constant regardless of total cathode emission. The means may be a second anode or a separate ring electrode and the control can be effected by acting on a control electrode close to the tip of the cathode.

United States Patent Smith et al.

ELECTRON GUNS Inventors: Kenneth Charles Arthur Smith, 50

Selwyn Rd., Cambridge; John Richard Adrian Cleaver, 27 Heath Rd, Potters Bar, Hertfordshire, both of England Filed: Apr. 24, 1973 Appl. No.: 353,979

US. Cl. 315/107, 313/82 R Int. Cl. H05b 39/04 Field of Search 313/81, 82 R, 82 BF;

References Cited UNITED STATES PATENTS H1963 Holmes 315/107 Mar. 18, 1975 3,197,634 7/1965 Walters ..315/107 3,309,557 3/1967 Crowley-Milling ..315/5 Primary ExaminerNathan Kaufman Attorney, Agent, or FirmScrivener Parker Scrivener and Clarke [57] ABSTRACT .In a field emission electron gun means are provided for monitoring the number of electrons that pass through the hole in the first anode of the gun and form the useful beam, these means being used to control the cathode emission so as to keep the useful beam current constant regardless of total cathode emission. The means may be a second anode or a separate ring electrode and the control can be effected by acting on a control electrode close to the tip of the cathode.

8 Claims, 1 Drawing Figure ti -Ir ELECTRON GUNS This invention relates to electron guns and to instruments, such as electron probe microanalysers, scanning electron microscopes, and electron beam welding equipment, which employ such guns.

The orthodox gun comprises a thermionic cathode and one or more anodes for accelerating the electrons and forming them into a beam, and these anodes may also have a focussing action on the beam, acting as a weak electrostatic lens. In the search for higher beam intensities in combination with smaller spot sizes it has been proposed over many years to use a field emission source in place of a thermionic source. However fieldemission sources require a much higher vacuum (of the order of Torr) than thermionic cathodes and also have contamination and erosion problems. Improvements in vacuum techniques in recent years, in combination with differential pumping of the electron gun to a higher vacuum than the remainder of the column, have largely overcome the first drawback but the second still presents serious problems.

lt is known in the case of thermionic electron guns to control the current in the heated filament, and the emission of electrons, in response to a feedback signal so that the emission is kept constant. It has also been proposed in the case of thermionic guns to check the whole beam current before or after use of the instrument and to use the result to adjust the filament current and hence the beam current. However this is hardly a practical procedure for day-to-day use and anyway it does not take account of changes in beam current that occur actually during use.

It has also been proposed to monitor the beam current at or near the surface of the specimen on which the beam impinges, and to use this signal to modify the heater current or to correct a signal derived from the specimen, but the use of a monitoring signal derived well down the column opens the way to errors due to a change in the monitoring signal resulting from a shift in beam alignment rather than a change in beam current.

Moreover in the case of field emission cathodes the problem is different, as the total electron current from the cathode by no means bears a constant relationship to the useful beam current, and the ratio of total to useful current changes during use as a result of erosion causing changes in the shape of the tip from which the emission takes place.

The aim of the present invention is to improve the stability and control of the electron beam from a field emission gun. According to the invention there is proposed a field emission gun comprising a field emission cathode and at least one anode and equipped with means to monitor the quantity of electrons falling on the anode, or one of the anodes, or on a control electrode placed in the path of electrons from the cathode to anode or anodes, in the immediate vicinity of the useful beam, the monitoring means producing a signal which is used to control the total emission from the cathode in a manner such as to keep the said quantity substantially constant.

By measuring the quantity of electrons falling on an electrode in the immediate vicinity of the path of the useful beam, or on an anode itself, we ensure that it is the useful beam current that it is kept constant, regardless of the total emission. It may will be that with changes in the shape of the tip of the cathode due to erosion, the total emission has to be increased at one time to keep the beam current at the desired value, yet I at another time the total emission has to be decreased for the same purpose.

Where there are two anodes each with beam-defining apertures, the result can be obtained by insulating the second anode and monitoring the current that flows to it. This gives a measure of the electrons that have passed through the aperture in the first anode but not the second, and this value is substantially proportional to the number that do pass through the second anode and form the useful beam.

Alternatively we may monitor the electrons falling on an auxiliary electrode in the form of a ring, through which the electrons pass, between the two anodes or follwoing the second anode or, where there is only one anode, following this anode.

The correction signal is preferably applied to a control electrode which is inserted between the cathode and the first anode, but much closer to the former than to the latter. This control electrode has a positive potential applied to it and provides the dominant influence on the potential gradient at the cathode surface, and thus on the current drawn. The tip of the cathode may be level with or protrude slightly through the aperture in the control electrode.

The invention will now be further described by way of example with reference to the accompanying drawing which is a diagramatic illustration of a gun according to the invention and its control circuit.

The gun comprises a cathode 1 in the form of a rod terminating in a pointed tip of very small radius, in a known manner, such that the electric field gradient in the immediate neighbourhood of the tip created by a first anode 2, positive with respect to the cathode, causes electrons to be emitted by the tip without any heating. However some heat may be applied by means not shown, in which case the emission is increased and operation is in what is known as the TF mode.

Unless a field emission cathode is operated under extremely good vacuum conditions there are normally significant fluctuations in the electron emission from its surface. Electron emission from the tip takes place over a large solid angle, with the local current density at a given point depending on crystal structure, surface roughness (which changes with time due to erosion) and on contamination. The useful beam, which passes through a hole 3 in the anode 2, consists only of electrons which have been emitted from a very small portion of the tip, close to the electron-optical axis A of the system. Fluctuations'in the emission at different regions of the tip are not correlated, and so the beam current is not directly related to the total current from the cathode.

The potential of the anode 2 influences the total emission and the focal properties of the gun. That part which passes through the hole 3 forms the useful beam and is accelerated to the required energy by a second anode 4 in which there is a hole 5. In a typical case the first anode 2 may be between 1 and 5 kilovolts positive with respect to the cathode, and the second anode is between 20 and 50 kilovolts positive. However in other situations, where only low-energy electrons are required, the second anode might even be at a lower voltage than the first, so that it retards the electrons.

Not all the electrons passing through the hole 3 pass also through the hole 5. On the contrary some strike the second anode 4 and the resulting current in the second anode is what we use to monitor the current. For this purpose the second anode 4, which is insulated from a surrounding enclosure by a ceramic ring 6, is connected to a current amplifier 7. The output of this amplifier acts through an electrically insulating link, indicated at 8, on means 9 that control the positive potential, with respect to the cathode l, of a control electrode 10. This electrode 10 is in the form of a disc with a central hole through which the tip of the cathode I nearly protrudes. lt strongly influences the electric field gradient at the tip, and therefore determines the emission of the cathode. The connections are such that any decrease or increase in the useful beam current, indicated by an increase in the current to the second anode 4, causes an increase or a decrease in the emission to restore the beam current to the desired value, set by a reference voltage supplied to the amplifier 7 at 11.

The purpose of the electrically insulating control link 8 is to allow for the fact that the anode 4 is at a very different potential from the cathode 1 and control electrode 10. It may take the form of an optical path. For example the amplifier 7 may control the brightness of a lamp or a light-emitting diode, the light from which falls on a photo-transistor controlling the potential on the electrode 10.

In practice, as is usual in electron guns, we put the second anode 4 at earth potential and the cathode 1 and its associated components are at a high negative potential, as indicated diagrammatically at 12.

Instead of monitoring the current in the second anode 4 we could monitor the current in a specially provided ring electrode following it, as indicated at 13 in the drawing, or between the two anodes as shown at 13. In some cases there may only be one anode, in which case we would have an electrode such as 13 following that anode.

In the example shown, the hole in the second anode 4 is small enough to allow differential pumping, i.e. to allow the region, indicated at U, containing the electron gun to be pumped out to a higher vacuum than the region V ofthe remainder of the instrument in which the gun is used.

It will be appreciated that bythe invention we have provided a compact self-regulating constant beamcurrent field emission gun, in which the regulation is achieved by monitoring means, either an anode or a separate ring electrode, associated with the gun itself, and sensing a signal closely dependent on the actual beam current, and not influenced by the alignment of the following electron optical system.

We may include in the gun some heating coils 14, shown fitting in apertures in the control electrode 10. These are not operative when the instrument is in normal use but when the machine is out of use they can be connected to a source of current to heat the surface of the first anode 2 and simultaneously bombard it with electrons, so as to clean and outgas that anode. The coils 14 may also be used to regenerate the cathode, with all electrodes grounded, or a separate heater may be provided for this purpose. For the cleaning and outgassing function, in place of the fixed coils 14 mounted in the control electrode 10, we could use a coil mounted on a swinging arm and capable of being brought into a position between the electrode and the first anode 2, or between the first and second anodes 2 and 4.

In addition to monitoring the beam current we preferably also monitor the total emission of the cathode and arrange to shut the gun down when the emission exceeds a predetermined value. This is because in time the emission increases with time, due to damage caused by sputtering, and the effect is cumulative, so that eventually the cathode is destroyed. The total emission can be satisfactorily monitored by measuring the current flow to the first anode 2, which intercepts all but a fairly small fraction of it.

Alternative forms for the insulating link 8 include a transformer or a capacitor, if the error signal is modulated onto an alternating current carrier, or is in pulse coded form.

What we claim is:

1. A self-contained field emission electron beamforming gun'assembly comprising a field emission cathode having a small-radius tip, an anode spaced from said cathode, said anode having a hole therethrough, means applying a positive electric potential to said anode with respect to said cathode whereby to cause the emergence of electrons from said tip, some of said electrons passing through said hole in the form of a beam, means for monitoring the quantity of said electrons passing through said hole, and correcting means responsive to said monitoring means, said correcting means determining the emission of electrons from said tip such as to keep said quantity of electrons passing through said hole substantially constant.

2. The gun assembly set forth in claim 1 wherein said monitoring means comprise current sensing means sensing the electric current in said anode.

3. The gun assembly set forth in claim 2 including a second anode, said second anode having a hole therethrough and being disposed between said tip and said first-mentioned anode, the respective holes in said anodes being alined.

4. The gun assembly set forth in claim 1 wherein said monitoring means comprise a further electrode on the opposite side of said anode from said cathode, said further electrode being placed to receive a portion of said quantity of electrons passing through the hole in said anode.

5. The gun assembly set forth in claim 4 including a second anode, said second anode lying on the opposite side of said further electrode from said cathode and first-mentioned anode, said second anode having a hole therein alined with said hole in said first-mentioned anode.

6. The gun assembly set forth in claim 4 wherein said further electrode comprises a ring, said ring being placed so that said quantity of electrons passing through said hole in said anode pass through said ring.

7. The gun assembly set forth in claim 1 wherein said correcting means comprise a control electrode in the immediate vicinity of said tip and having an electric potential, with respect to said tip, which is variable such as to vary the potential gradient at said tip and thereby determine the electron emission of said cathode.

8. The gun assembly set forth in claim 7 wherein said control electrode comprises a disc having an opening therein, said tip of said fieldemission cathode lying in said opening. 

1. A self-contained field emission electron beamforming gun assembly comprising a field emission cathode having a smallradius tip, an anode spaced from said cathode, said anode having a hole therethrough, means applying a positive electric potential to said anode with respect to said cathode whereby to cause the emergence of electrons from said tip, some of said electrons passing through said hole in the form of a beam, means for monitoring the quantity of said electrons passing through said hole, and correcting means responsive to said monitoring means, said correcting means determining the emission of electrons from said tip such as to keep said quantity of electrons passing through said hole substantially constant.
 2. The gun assembly set forth in claim 1 wherein said monitoring means comprise current sensing means sensing the electric current in said anode.
 3. The gun assembly set forth in claim 2 including a second anode, said second anode having a hole therethrough and being disposed between said tip and said first-mentioned anode, the respective holes in said anodes being alined.
 4. The gun assembly set forth in claim 1 wherein said monitoring means comprise a further electrode on the opposite side of said anode from said cathode, said further electrode being placed to receive a portion of said quantity of electrons passing through the hole in said anode.
 5. The gun assembly set forth in claim 4 including a second anode, said second anode lying on the opposite side of said further electrode from said cathode and first-mentioned anode, said second anode having a hole therein alined with said hole in said first-mentioned anode.
 6. The gun assembly set forth in claim 4 wherein said further electrode comprises a ring, said ring being placed so that said quantity of electrons passing through said hole in said anode pass through said ring.
 7. The gun assembly set forth in claim 1 wherein said correcting means comprise a control electrode in the immediate vicinity of said tip and having an electric potential, with respect to said tip, which is variable such as to vary the potential gradient at said tip and thereby determine the electron emission of said cathode.
 8. The gun assembly set forth in claim 7 wherein said control electrode comprises a disc having an opening therein, said tip of said field emission cathode lying in said opening. 